IN RATS AND GUINEA-PIGS. II 



The gray Irish female (141) has produced, by hooded males, young of the 

 Irish pattern only, 19 in number, some of them being gray, and others black 

 pigmented; she has also produced albino young by hooded males bearing 

 recessive albinism. She accordingly bears black pigmentation and total 

 albinism as recessive characters, but is otherwise homozygous, for if she bore 

 the hooded pattern, half her pigmented young should have been hooded. 

 By similar tests, the gametic formulae of individuals 142 to 145 were estab- 

 lished, as given in fig. i. 



Experiments such as these show the complete mutual independence of 

 pigmentation, color-pattern, and total albinism, as has been repeatedly 

 stated. They indicate, further, that partial albino rats (hooded and Irish) 

 regularly form gametes which bear the partial albino condition, either in 

 part or in all of their gametes; and that the extent of the white areas on 

 partial albinos varies in continuous fashion, a self condition being some- 

 times obtained as an extreme variation of Irish. This last point is impor- 

 tant as showing that no complete discontinuity exists between self and 

 Irish patterns in rats, since the gap ordinarily existing between them can 

 be bridged with a complete series of intermediate conditions. The same is 

 true of the Irish and hooded conditions. Though ordinarily discontinuous 

 and behaving as alternative Mendelian characters in inheritance, we can by 

 cross-breeding and selection bridge the gap between them. 



MODIFICATION OF HOODED PATTERN BY CROSSING WITH IRISH. 



Hooded and Irish individuals differ, as already stated, simply in the 

 extent of their pigmentation. In hooded individuals, the pigmentation is 

 restricted to the hood (which covers the head and shoulders) and to the 

 dorsal stripe; in Irish individuals, the entire coat is pigmented except a 

 ventral patch of variable size. Extension of the dorsal stripe of the hooded 

 pattern, so as to cover the entire dorsal surface and the sides of the body, 

 would yield the Irish pattern. It was our purpose in one set of experiments 

 to see whether such modification could be brought about, and to see also 

 how far an opposite modification of the hooded pattern (further reduction 

 of the pigmented areas) could be effected. Before considering those experi- 

 ments in detail, we may inquire what effect upon the hooded pattern a cross 

 with the Irish pattern will have. 



The hooded stock used in this investigation consisted of individuals which 

 bore albinism recessive, but had no Irish ancestors, so far a? known. The 

 extent of the dorsal stripe varied considerably, so that it was found desir- 

 able to measure it as accurately as possible in each individual. This was 

 done, first, by estimating the width of the stripe in relation to the total 

 width of the back; next was estimated the extent to which the stripe was 



